1. Field of the Invention
The present invention relates to vibrational gyroscopes, that is to say rotation sensors of the type comprising a member driven to vibrate and provided with means for sensing vibrational changes resulting from the action of Coriolis forces on the vibrating member when it is rotated about a sensitive axis.
2. Description of the Related Art
Vibrational gyroscopes have many advantages over conventional gyroscopes of the spinning wheel type. Thus a vibrational gyroscope is considerably more rugged than a conventional spinning wheel gyroscope, can be started up much more quickly, consumes much less power and has no bearings which could be susceptible to wear.
A wide variety of vibrating members have been employed in previously proposed vibrational gyroscopes, ranging in shape from a tuning fork to a pair of torsionally oscillating coaxial spoked wheels. However the present invention is particularly concerned with vibrational gyroscopes in which the vibrating member comprises a radially vibrating annular shell, such as a hemispherical bell or a cylinder for example. In such gyroscopes the axis of the annular shell(say the z axis)is the sensitive axis and the shell, when vibrating, periodically distorts in an elliptical fashion with four nodes spaced regularly around the circumference and located on the x and y axes. Any rotation about the z axis generates tangential periodic Coriolis forces which tend to shift the vibrational nodes around the circumference of the shell and thereby generate some radial vibration at the original nodal positions on the x and y axes. Consequently the output of one or more transducers located at one or more of these nodal positions gives a measure of the rotation rate (relative to an inertial frame) about the z-axis.
This highly symmetrical system has a number of important advantages over arrangements in which the vibrating member is not rotationally symmetrical about the z-axis. Thus the component of vibration rotationally induced by the Coriolis forces is precisely similar to the driving vibration. Consequently if the frequency of the driving vibration changes (e.g. due to temperature variations) the frequency of the rotationally induced component of vibration will change by an identical amount. Thus if the amplitude of the driving vibration is maintained constant the amplitude of the rotationally induced component will not vary with temperature. Also the elliptical nature of the vibrational distortion ensures that the instantaneous polar moment of inertia about the z-axis is substantially constant throughout each cycle of the vibration. Consequently any oscillating torque about the z-axis (due to externally applied rotational vibration) will not couple with the vibration of the walls of the shell. Accordingly vibration gyroscopes incorporating an annular shell as the vibrating member offer superior immunity to temperature changes and external vibration.
However in practice, vibrational gyroscopes generally employ piezoelectric transducers both for driving and sensing the vibration of the vibrating member. In cases where a vibrating annular shell is employed, the transducers are mounted on the curved surface of the shell, generally near its rim. Consequently the axial symmetry of the annular shell is lost, so that the resonant frequency in the sense direction is no longer the same as the driving frequency. In order to obtain a reasonable response, this resonant frequency must be adjusted to equal the drive frequency, typically by removing material from parts of the rim of the annular shell, which is a difficult operation. Furthermore, since it is difficult to form a low compliance bond between two curved surfaces, the transducers must be sufficiently small to form an essentially flat interface with the curved surface of the annular shell. The output of the vibration-sensing transducers is limited by their strain capability, so that the sensitivity of the system is limited by signal-to-noise ratio. All these problems become more acute as the dimensions of the annular shell are reduced.